This invention relates to an improved no moving parts horizon sensor, and more particularly, to such a sensor which detects and provides attitude information in response to the intensity of visible and ultraviolet radiation from the crescent or fully illuminated earth disc produced by scattered radiation from the sun.
Horizon sensors are commonly used in satellites and space craft for maintaining level flight or providing predetermined attitudes of components mounted or positioned in the host vehicle. The horizon sensor operates by sensing the sharp line of thermal discontinuity which is presented between the earth's horizon and outer space. Horizon sensors primarily utilized infrared radiation by comparing the practically zero level of infrared radiation in space with the enormous or substantial quantity of infrared radiation emitted by the earth. Detectors are provided which are responsive to such radiation and these detectors are scanned successively across the earth to outer space or vice versa and repeatedly provide horizon crossing information in the form of an electrical signal identifying the sharp line of thermal discontinuity. Various types of scanning mechanisms have been employed, for example, rotating mirrors, rotating prisms, the rotation of the satellite which rotates a fixed detecting means, as well as scanners in the form of detector arrays which are self scanned electronically.
In U.S. Pat. No. 3,118,063 horizon sensors were provided which were selectively responsive to infrared emission from a gaseous component in the atmosphere. In other words, radiation from the earth or the clouds either reflected or semi-emitted was not used which was a considerable improvement in horizon sensoring accuracy because the sensor was blind to radiation reflected from the clouds. Accordingly, no problem was presented by a false horizon caused by such radiation. Furthermore, the emission from the earth's crust is not uniform and varies depending on the location and the climatic conditions. However, the use of this gaseous component requires an infrared detector whose radiation must be chopped or interrupted before it is applied to a detector so that the detector can respond and generate a signal. The chopping mechanism requires moving parts which are a disadvantage with respect to operational life in space. The possibility of mechanical malfunction, as well as the power requirements to drive the chopper which adds to the weight required for the sensor are added disadvantages.